Reference circuits may be used in a variety of applications to provide known reference values, such as reference voltages or currents. For example, a bandgap voltage reference circuit may provide a temperature independent voltage reference for use with other circuits such as flash memory circuits, other memory circuits, and/or other integrated circuits. Reference circuits may include voltage adjusting circuits, for example startup circuits. A voltage adjusting circuit may initialize one or more inputs of another circuit by forcing a voltage on a node or a current into a branch, for example. This may allow the circuit connected to the voltage adjusting circuit to quickly begin operation in a proper initial state. For example, in the voltage adjusting circuits described herein, a reference voltage may be applied to a feedback node by a voltage adjusting circuit when a voltage at a detection node is not a desired voltage.
It may be desirable to design voltage reference circuits with low current consumption so that their presence does not adversely affect operation of an associated circuit. Voltage adjusting circuits described herein may enable the creation of cost efficient, low current consumption reference circuits.
FIGS. 1A and 1B are prior art voltage adjusting circuits 100, 105. Voltage adjusting circuit 100 of FIG. 1A is a pull down type voltage adjusting circuit, and voltage adjusting circuit 105 of FIG. 1B is a pull up type voltage adjusting circuit. The voltage adjusting circuits 100, 105 may communicate with a reference circuit 10 at a feedback node 120 and a detection node 125. The reference circuit 10 may be any circuit having an operation which can be improved by a voltage adjusting circuit such as a startup circuit. Examples include the reference circuits disclosed in H. Banba, et al., “A CMOS Band Gap Reference Circuit With Sub-1-V Operation,” IEEE Journal of Solid-State Circuits, Vol. 34, No. 5, May 1999, pp. 670-674, the contents of which are incorporated herein by reference in their entirety. The detection node 125 may represent the output of the reference circuit 10 and the feedback node 120 may represent the input to an element controlling the output of the reference circuit.
The pull down voltage adjusting circuit 100 of FIG. 1A may include a resistive load common source amplifier Mamp 110, for example a transistor with its source connected to ground 145 and its drain connected to a node Vgate 130. A resistor 160, having a value of Rstartup, may be connected between a voltage source Vdd 140 and Vgate 130. The gate of the amplifier Mamp 110 may be connected to the detection node 125. A pull down transistor 150 may have its drain connected to the feedback node 120, its gate connected to Vgate 130, and its source connected to ground 145. The amplifier Mamp 110 may drive the pull down transistor 150 to pull the feedback node 120 out of an undesirable metastable or unstable operation of reference circuit 10 when the feedback node 120 is not at the desired voltage. Thus, the detection node 125 indicates two stable states of the reference circuit 10, an active state and a non-active state. When the detection node indicates a non-active state, the voltage adjusting circuit 100 applies a voltage to the feedback node 120 to cause the reference circuit 10 to enter the active state.
Similarly, the pull up voltage adjusting circuit 105 of FIG. 1B may include a resistive load common source amplifier Mamp 110, for example a transistor with its drain connected to a node Vgate 130 and its source connected to a voltage source Vdd 140. A resistor 160, having a value of Rstartup, may be connected between Vgate 130 and ground 145. The gate of the amplifier Mamp 110 may be connected to the detection node 125. A pull up transistor 155 may have its source connected to the voltage source 140, its gate connected to Vgate 130, and its drain connected to the feedback node 120. The amplifier Mamp 110 may drive the pull up transistor 150 to pull the feedback node 120 to a desired reference voltage when the feedback node 120 is not at the desired voltage.
The current consumption of the prior art voltage adjusting circuit examples 100, 105 may be approximated by Vdd/Rstartup, because an equivalent resistance of Mamp 110 may be substantially smaller than the resistance Rstartup of resistor 160. In order to keep the current consumption small, the value Rstartup of resistor 160 may be high. Thus, these circuits 100, 105 may use expensive resistors with high resistance values and/or large resistor arrays. For example, in a case wherein Vdd is 3V, and the desired standby current for the voltage adjusting circuit is 1 μA or less, a 3 MΩ resistor may be used.
The circuits of FIGS. 1A and 1B may be employed as startup circuits.